Chapter 1

Introduction to Spectroscopy

Ephrem Tilahun (MSc., Ass.Prof)

Jimma University

1 E.T

• Spectroscopy is the study of the interaction between radiation (electromagnetic radiation or light, as well as particle radiation) and matter.

• Spectrometry is the measurement of these interactions

• instrument which performs such measurement

– spectrometer or

– spectrograph.

• A plot of the interaction is referred to as a

– spectrum

2 E.T

Classification of spectroscopic methods

• Nature of radiation measured

• The type of spectroscopy depends on the physical quantity measured.

• Normally, the quantity that is measured is an amount or intensity of something.

• Electromagnetic spectroscopy involves interactions with electromagnetic radiation, or light. – Ultraviolet-visible spectroscopy is an example.

• Electronic spectroscopy involves interactions with electron beams. – Auger spectroscopy involves inducing the Auger effect with an electron

beam.

• Mechanical spectroscopy involves interactions with macroscopic vibrations, such as phonons. – An example is acoustic spectroscopy, involving sound waves.

• Mass spectroscopy involves the interaction of charged species with magnetic and/or electric fields, giving rise to a mass spectrum.

3 E.T

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• Measurement process

• Most spectroscopic methods are differentiated as either atomic or molecular

based on whether or not they apply to atoms or molecules.

• Along with that distinction, they can be classified on the nature of their

interaction:

• Absorption spectroscopy uses the range of the electromagnetic spectra in

which a substance absorbs.

– atomic absorption spectroscopy and

– various molecular techniques, such as infrared spectroscopy in that region a

– nuclear magnetic resonance (NMR) spectroscopy in the radio region.

• Emission spectroscopy uses the range of electromagnetic spectra in which a

substance radiates (emits).

– luminescence.

– Molecular luminescence techniques include spectrofluorimetry.

• Scattering spectroscopy measures the amount of light that a substance

scatters at certain wavelengths, incident angles, and polarization angles.

– The scattering process is much faster than the absorption/emission process.

• Raman spectroscopy

4 E.T

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Electromagnetic radiation

• EMR, or light, is a form of energy whose behaviour is described by the properties of both waves and particles.

• The optical properties of electromagnetic radiation, such as diffraction, are explained best by describing light as a wave.

• Many of the interactions between electromagnetic radiation and matter, such as absorption and emission,

– however, are better described by treating light as a particle, or photon.

• the dual models of wave and particle behaviour provide a useful description for electromagnetic radiation

5 E.T

• Wave Properties of Electromagnetic Radiation

• Electromagnetic radiation consists of oscillating electric and magnetic fields that propagate through space along a linear path and with a constant velocity

• In a vacuum, electromagnetic radiation travels at the speed of light, c, which is 2.99792 x108 m/s.

• Electromagnetic radiation moves through a medium other than a vacuum with a velocity, v, less than that of the speed of light in a vacuum.

• Oscillations in the electric and magnetic fields are perpendicular to each other, and to the direction of the wave’s propagation.

6 E.T

Figure shows an example of plane-polarized electromagnetic radiation consisting of

an oscillating electric field and an oscillating magnetic field, each of which is

constrained to a single plane. 7 E.T

• The interaction of electromagnetic radiation with matter can be

explained using either the electric field or the magnetic field.

• For this reason, only the electric field component is shown in

Figure

8 E.T

• The oscillating electric field is described by a sine wave of the form

E = Ae sin(2πνt + Φ)

– where E is the magnitude of the electric field at time t, Ae is the electric field’s

maximum amplitude,

– v is the frequency, or the number of oscillations in the electric field per unit time, and

– Φ is a phase angle accounting for the fact that the electric field’s magnitude need not be zero at t = 0.

• An identical equation can be written for the magnetic field, M

M = Am sin(2πνt + Φ)

– where Am is the magnetic field’s maximum amplitude.

• only' the electrical vector is employed because it is the electrical force that is responsible for such phenomena as transmission, reflection, refraction, and absorption of radiation.

• Magnetic responsible for the absorption of radio-frequency waves in NMR

9 E.T

Wave Parameters

• The time required for the passage of successive maxima through a fixed point in space is called the period (p )of the radiation.

• The frequency (v) is the number of oscillations of the field that occurs per second and is equal to 1/p.

• Another parameter of interest is the wavelength λ, which is the linear distance between any two equivalent point on successive maxima or minima of a wave.

• Multiplication of the frequency in reciprocal seconds by the wavelength in metres gives the velocity of propagation

10 E.T

V= vλ

• It is important to realize that the frequency is determined by the source and remains invariant regardless of the media traversed by the radiation.

• In contrast, the velocity of propagation v, the rate at' which the wave front moves through a medium, is dependent upon both the medium and the frequency;

• In a vacuum the velocity of radiation becomes independent of frequency and is at its maximum.

• This velocity, given the symbol c, has been accurately determined to be 2.99792 X 1010 cm/s. Thus, for a vacuum,

• c= νλ = 3 x 1010 cm/s

11 E.T

• In any other medium, the rate of propagation is less because of

interactions between the electromagnetic field of the radiation

and the bound electrons in the atoms or molecules making up

the medium.

• Since the radiant frequency is invariant and fixed by the

source, the wavelength must decrease as passes from a

vacuum to a medium containing matter.

wave number, , which is the reciprocal of wavelength 12 E.T

• Particle Properties of Electromagnetic Radiation

• An understanding of certain interactions between radiation and

matter requires that the radiation be treated as packets of

energy called photons or quanta.

– The energy of a photon depends upon the frequency of the radiation,

and is given by

where h is Planck’s constant, which has a value of 6.626 x10–34 J · s.

Photon : A particle of EMR having zero mass and an energy of hv.

13 E.T

Example 1:- The “D” line in the solar spectrum was due to the

absorption of solar radiation by sodium atoms. The wavelength of

the sodium D line is 589 nm. What are the frequency and the wave

number for this line?

SOLUTION

The frequency and wave number of the sodium D line are

14 E.T

Example 2:- What is the energy per photon of the sodium D line

(λ= 589 nm)?

SOLUTION

The energy of the sodium D line is

15 E.T

Electromagnetic (EM) spectrum

• The electromagnetic (EM) spectrum is just a name that scientists give a bunch of types of radiation when they want to talk about them as a group.

• Radiation is energy that travels and spreads out as it goes

– visible light that comes from a lamp in your house or

– radio waves that come from a radio station

• Other examples of EM radiation are microwaves, infrared and ultraviolet light, X-rays and gamma-rays.

16 E.T

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17 E.T

• I. Absorption of Radiation

• When radiation passes through a transparent medium contain

chemical species, certain frequencies may be selectively

attenuate by the process of absorption.

• EMR energy is transferred to the atoms or molecules

constituting the sample; as a result, these particles are

promoted from a lower energy state to higher-energy states, or

excited states.

• At room temperature, most substances are in their lowest

energy or ground state.

• Absorption then ordinarily involves a transition from the

ground state to higher-energy states.

• Atoms, molecules, or ions have only at limited number of

discrete, quantized energy levels;

18 E.T

• for absorption of radiation to occur, the energy

of the exciting photon must exactly match the

energy difference between the ground state and

one of the excited states of the absorbing

species.

• consider the three processes that may occur

when a two-state system is subjected to

radiation of frequency or wave number ,

corresponding to the energy separation ∆E

where

19 E.T

1. Induced absorption, in which the molecular (or atom) M absorbs

a quantum of radiation and is excited from m to n:

Eg. the appearance of an aqueous solution of copper sulphate as blue

due to the absorption of the complementary colour, red, by the solution.

2. Spontaneous emission, in which M* (in state n) spontaneously

emits a quantum of radiation:

Eg. a sodium vapour or tungsten filament lamp, is of the

spontaneous type. 20 E.T

3. Induced, or stimulated, emission.

This is a different type of emission process from that of type 2 in that

a quantum of radiation of wave number is required to induce,

or stimulate, M* to go from n to m.

The process is represented by

Since the energy d/n are unique for each species, a study of the

frequencies of absorbed radiation provides a means of characterizing

the constituents of a sample of matter.

For this purpose, a plot of absorbance as a function of wavelength or

frequency is experimentally derived absorption spectra

21 E.T

The general appearance of an absorption spectrum will depend upon

the complexity,

the physical state, and

the environment of the absorbing species.

It is convenient to recognize two types of spectra, namely, those

associated with

atomic absorption and

those resulting from molecular absorption.

22 E.T

• A. Atomic Absorption

• When a beam of polychromatic Uv or Vis

radiation passes through a medium containing

gaseous atoms, only a few frequencies are

attenuated by absorption, and

• The spectrum consists of a number of very

narrow absorption lines

23 E.T

200 300 400 500 600 Wavelength, nm

A

3s

3p

4p

5p

E

59

0

33

0

28

5

0

1

2

3

4

(a)Absorption spectrum for Na vapour (b) partial energy level diagram for Na

Transitions involves excitation of

the single outer electron of Na

24 E.T

• B. Molecular Absorption

• Molecules undergo three type of quantized transitions when

exited by UV, visible and IR radiation.

• For UV and VIS radiation excitation involves promoting an

electron from ground state (low energy molecular or atomic

orbital )to exited state (a higher – energy orbital).

• The energy hv of the photon must be exactly the same as the

energy d/n b/n the two orbital energies.

• The transition known as Electronic transition

– electronic absorption

• Molecules also exhibit two other types of radiation- induced

transitions

– Vibrational

– Rotational

•

25 E.T

• For each electronic energy state of the molecule, there

normally exist several possible vibrational states, and for

each of these, in turn, numerous rotational states,

26 E.T

Symmetric Asymmetric

A)Stretching Vibrations

In- plane scissoring

- + + +

- In-plane rocking Out-plane wagging Out-plane twisting

B) Bending vibrations

Vibrational transition come about b/c a molecule has a multitude of

quantized energy levels (vibrational state) associated with the bond that

holed the molecule together.

27 E.T

• Absorption by polyatomic molecules, particularly in the

condensed state, is a considerably more complex process

because the number of energy states is greatly enhanced.

• Here, the total energy of a molecule is given by

• E =Eele + Evib. + Erot.

– where Eele describes the electronic energy of the molecule, and

– Evib refers to the energy of the molecule resulting from various

atomic vibrations.

– The third term accounts for the energy associated with the

rotation of the molecule around its center of gravity.

28 E.T

Figure below is a graphical representation of the energies

associated with few of the electronic and vibrational states of a

molecule.

Fig. 29 E.T

• The heavy line labelled Eo represents the electronic

energy of the molecule in its ground state (its state of

lowest electronic energy);

• the lines labelled E1 and E2 represent the energies of

two excited electronic states.

• Several vibrational energy levels (0,1,…,4) are shown

for each of these electronic states.

• the energy difference between the ground state and an

electronically excited state is large relative to the

energy differences b/n vibration levels in a given

electronic state

– (typically, the two differ by a factor of 10 to 100).

30 E.T

• Visible radiation causes excitation of an electron from

Eo to any of the vibration levels associated with E 1'

• Ultraviolet causes excitation of an electron from Eo to

any of the vibration levels associated with E2

• Finally, the less energetic near- and mid infrared

radiation can only bring about transition among the

vibrational levels of the ground state.

• Although they are not shown, several rotational

energy levels are associated with each vibrational

level.

• The energy difference between these is small relative

to the energy difference between vibrational levels;

31 E.T

In contrast to atomic absorption spectra, which consist of

a series of sharp, well-defined lines,

Molecular spectra in the ultraviolet and visible regions are

ordinarily characterized by absorption bands

that often encompass a substantial wavelength the spectrum for a

molecule ordinarily consists of a series of closely spaced

absorption bands, such as that shown for benzene vapor in Figure

below

32 E.T

Unless a high-resolution instrument is employed, the

individual bands may not be detected, and the spectra will

appear as smooth curves.

Finally, in the condensed state and in the presence of

solvent molecules, the individual bands tend to broaden to

give spectra such as those shown in the Figure below

33 E.T

• Pure vibrational absorption can be observed in the

infrared region.

– where the energy of radiation is insufficient to cause

electronic transitions.

• Variations in rotational levels may give rise to a series

of peaks for each vibrational state.

– However, rotation is often hindered or prevented in liquid

or solid samples;

– the effects of these small energy differences are

not ordinarily detected in such samples.

34 E.T

• Pure rotational spectra for gases can be observed in the

microwave region.

– Absorption Induced by a Magnetic Field.

• When electrons or the nuclei of certain elements are

subjected to a strong magnetic field additional quantized

energy levels are produced as a consequence of magnetic

properties of these elementary particles.

– Absorption by nuclei or by electrons in magnetic fields is

studied by nuclear magnetic resonance (NMR) and

– electron spin resonance (ESR) techniques, respectively

35 E.T

Relaxation Processes

• Ordinarily, the lifetime of an atom or molecule

excited by absorption of radiation is short because

several relaxation processes exist which permit its

return to the ground state.

• Two of the most important of these mechanisms

• nonradiative relaxation

• fluorescent relaxation

36 E.T

E.T 37

Fig. 5

• Two type of nonradiative relaxation are shown in the figure 5b

Vibrational deactivation or relaxation,

– denoted by short arrows b/n vibrational energy levels,

– involves the loss of energy in a series of small steps,

– the excitation energy being converted to kinetic energy by collision with

solvent molecules.

• These energy transferd to solvent molecule and reflacted by a

tiny increase in the temperature of the medium.

• Non radiative relaxation b/n the lowest vibrational level of an

exited electronic state and the upper vibrational level of another

electronic level can also occur.

– Represent by long arrow.

• Less efficient than vibrational relaxation and its life time is b/n

10-6 and 10-9

• As shown in Figure 5c, relaxation can also occur by emission of

fluorescent radiation.

38 E.T

II. Emission Radiation

• Excited particles (ions, atoms, or molecules)can be excited to

one or more higher energy levels by a variety of means,

including

– bombardment with electrons or other elementary particles,

– exposure to a high-potential alternating current spark,

– heat treatment in an arc or a flame, or

– absorption of electromagnetic radiation.

• The lifetime of an excited species is generally transitory (10-6

– 10-9 s) and

• a relaxation to a lower-energy levels or to their ground states

takes place with the release of the excess energy in the form of

Electromagnetic radiation, heat or perhaps both.

39 E.T

• Emission spectra

• A plot of the relative power of the emitted radiation

as a function of wavelength or frequency.

40 E.T

Figure 6 Emission spectrum of a brine sample obtained with an oxyhydrogen flame. The spectrum consists of the superimposed line. band. and continuum spectra of the constituents of the sample. The character- istic wavelengths of the species contributing to the spectrum are listed beside each feature. (R. Hermann and

C. T. J. Alkemade, Chemical Analysis by Flame Photometry, 2nd ed.. p. 484. New York: Interscience, 1979.)

• There are three type of spectra:- line, band and

continues.

• Excitation can be brought about Radiating particles

that are well separated from one another, as in the

gaseous state, behave as independent bodies and often

produce radiation containing relatively few specific

wavelengths.

• The resulting spectrum is then discontinuous and is

termed a line spectrum.

• Made up of a series of sharp, well-defined peaks

caused by an excitation of individual atom.

• In fig. 6 lines for Na, K, Sr and Ca

41 E.T

• The band spectrum consists of several groups of lines so

closely spaced that they are not completely resolved.

– The source of the band is small molecules or radicals

– In fig.6 bands of OH,MgOH and MgO

• Bands arise from the numerous quantized vibrational levels

that are superimposed on the ground state electronic energy

level of a molecule.

• A continuous spectrum, on the other hand, is one in which all

wavelengths are represented over an appreciable range, or one

in which the individual wavelengths are so closely spaced that

resolution is not feasible by ordinary means.

– The line and band spectra are superimposed

• Continuous spectra result from excitation of:

• (1) solids or liquids, in which the atoms are so closely packed

as to be incapable of independent behaviour; or

• (2) complicated molecules possessing many closely related

energy states.

42 E.T

• When solids are heated to incandescence, the continuous

radiation that is emitted is more characteristic of the

temperature of the emitting surface than of the material of

which that surface is composed.

• Radiation of this kind (called black-body radiation)

• is produced by the innumerable atomic and molecular

oscillations excited in the condensed solid by the thermal

energy.

43 E.T

Figure 7 Blackbody radiation curves for various light sources. Note the shift in the peaks as the temperature of the sources changes.

• Both continuous spectra and line spectra are of

importance in analytical chemistry.

• The former are frequently employed in

methods based on the interaction of radiation

with matter, such as spectrophotometry.

• Line spectra, on the other hand, are important

because • they permit the identification and determination of the

emitting species.

44 E.T

Absorption Law

Chapter 2

E.T 45

The absorption law

• also known as the Beer-Lambert law or just Beer's law,

• Tells us quantitatively how the amount of attenuation depends on the concentration of the absorbing molecules and the path length over which absorption occurs.

• As light traverses a medium containing an absorbing analyte, decreases in intensity occur as� the analyte becomes excited.

E.T 46

Transmittance and Absorbance

• The common terms and alternative names/symbols

employed in spectroscopy are listed in the table

below. The recommended terms and symbols are

listed under the column labeled Term and Symbol.

E.T 47

Terms and Symbols

E.T 48

Transmittance

E.T 49

Figure 1 depicts a beam of parallel radiation before and after it has passed

through a layer of solution with a thickness of b cm and a concentration of c of

an absorbing species.

As a consequence of interactions between the photons and absorbing particles,

the power of the beam is attenuated from Po to P.

The transmittance T of the solution is the fraction

Equ.1

Absorbance

• Absorbance, like the previous table shows, can

be defined as the base-ten logarithm of the

reciprocal of the transmittance :

A = log 1/T = -log I/Io = -log P/Po

E.T 50

Equ.2

• Absorptivity and Molar Absorptivity

• Absorbance is directly proportional to the path length through the solution and the concentration of the absorbing species.

– That is, A = abc

• where a is a proportionality constant called – the absorptivity.

• The magnitude will clearly depend upon the units used for b(cm) and c (g/L).

• When the concentration is expressed in moles per litter and the cell length is in centimetres,

– the absorptivity is called the molar absorptivity and given the special symbol ԑ

– A = ԑbc

E.T 51

Experimental Measurements of Absorbance and

transmittance

• The relationship given by Equation 1 and 2 is not directly

applicable to chemical analysis.

• Neither Absorbance nor Transmittance, as defined, can be

conveniently measured in the laboratory

– because the solution to be studied must be held in some sort of

container.

• Interaction between the radiation and the walls is inevitable

leading to a loss by reflection at each interface;

• significant absorption may occur within the walls themselves.

• the beam may suffer a diminution in power during its passage

through the solution as a result of scattering by large

molecules or inhomogeneities. E.T 52

Reflection and Scattering Losses

E.T 53

Reflection and Scattering Losses (cont)

• Reflection and scattering losses are significant and to compensate for these effects, the power of the beam transmitted by the analyte solution is ordinarily compared with the power of the beam transmitted by an identical cell containing only the solvent.

• An experimental absorbance that closely approximates the true absorbance is then obtained with the equation:

A = log Psolvent/Psolution ≈ log Po/P

E.T 54

• Example:

• A sample in a 1.0 cm cell is determined with a

spectrometer to transmit 80 % light at a certain

wavelength. If the absorptivity of this substance at

this wavelength is 2.0, what is the concentration of

the substance?

• Solution:

• The percent transmittance is 80 % and so T = 0.80:

E.T 55

-1 11log 2.0 cm 1.0 cm0.80

g L c

-1log1.25 2.0 g L c

0.100.050 /

2.0c g L

E.T 56

0.70T 1

log log1.43 0.1550.70

A

0.155 (0.0100 / )ab g L

15.5 /ab L g15.5 L/g (4 0.0100 g/L) = 0.620A

1log 0.620

T

0.240T 1 1 1

2 2 2

A abc c

A abc c

22 1

1

40.155 0.620

1

cA A

c

Solution:

Therefor

e,

The absorbance of the new solution could have

been calculated more directly:

Example:

A solution containing 1.00 mg ion (as the thiocyanate complex) in 100 mL

was

observed to transmit 70.0% of incident light compared to an appropriate

blank.

(a) What is the absorbance of the solution at this wavelength?

(b) What fraction of light would be transmitted by a solution of iron four

times as concentrated?

Beer’s Law

Consider This:

• A parallel beam of monochromatic radiation with power Po strikes the block perpendicular to a

surface after passing through a length b of the material , which contains n absorbing particles , the beam’s power is decreased to P as a result of absorption.

E.T 57

an

area S and an infinitesimal thickness dx.

Within this section there are dn absorbing

particles;

associated with each particle, we can imagine a

surface

at which photon capture will occur.

That is, if a photon reaches one of these areas by

chance, absorption will follow immediately.

The total projected area of these capture

surfaces

within the section is designated as dS;

the ratio of the capture area to the total area,

then, is

dS/S.

On a statistical average, this ratio represents the

probability for the capture of photons within the

E.T 58

The power of the beam entering the section, Px,

is proportional to the number of photons per

square centimeter per second, and

dPx represents the quantity removed per second

within the section;

the fraction absorbed is then - dPx /Px, and this

ratio also equals the average probability for

capture.

The term is given a minus sign to indicate that P

undergoes a decrease. Thus,

eq.3

E.T 59

Recall, now, that dS is the sum of the capture areas for particles within the

section; it must therefore be proportional to the number of particles, or

where dn is the number of particles and a is a proportionality

constant, which can be called the capture cross section

Combining Equations 3 and 4 and summing over the interval

between zero and n, we obtain

Integration gives

eq 6

eq. 4

eq 5

E.T 60

converting to base 10 logarithms and inverting the fraction to

change the sign, we obtain

eq 7

where n is the total number of particles within the block shown in Figure 2. The

cross sectional area S can be expressed in terms of the volume of the block V

and its length b.

Thus,

Substitution of this quantity into Equation 7 yields

eq. 8

E.T 61

Note that n/V has the units of concentration (that is, number of particles per

cubic centimetre); we can readily convert n/V to moles per litter. Thus, number

of moles given by

Combining eq.8 and 9

eq.9

E.T 62

and c in mol/L is given by

E.T 63

Finally, the constants in this equation can be collected into a

single term ԑ to give

which is Beer's law.

Application of Beer`s Law to

Mixture

• Beer's law also applies to a solution containing more than one

kind of absorbing substance.

• Provided there is no interaction among the various species, the

total absorbance for a multicomponent system is given by

Atotal = A1 + A2 + ... + An (6-11) = ԑ1bcl + ԑ2bc2 + ... + ԑnbcn

• where the subscripts refer to absorbing components 1, 2, ..., n.

E.T 64

Deviations from Beer’s Law

• Beer’s law states that a plot of absorbance versus

concentration should give a straight line passing

through the origin with a slope equal to ab. However,

deviations from direct proportionality between

absorbance and concentration are sometimes

encountered.

• These deviations are a result of one or more of the

following three things ; real limitations, instrumental

factors or chemical factors.

E.T 65

Real Limitations

• Beer’s law is successful in describing the absorption behavior of dilute solutions only ; in this sense it is a limiting law. At high concentrations ( > 0.01M ),the average distance between the species responsible for absorption is diminished to the point where each affects the charge distribution of its neighbors.

• This interaction, in turn, can alter the species’ ability to absorb at a given wavelength of radiation – Because the extent of interaction depends upon concentration,

• the occurrence of this phenomenon thus leading to a deviation from Beer’s law.

E.T 66

Limitations (cont)

• A similar effect is sometimes encountered in solutions

containing low absorber concentrations and high

concentrations of other species, particularly electrolytes.

• The close proximity of ions to the absorber alters the molar

absorptivity of the latter by electrostatic interactions;

– the effect is lessened by dilution

• Deviations also arise because ԑ is dependent upon the

refractive index of the solution. Thus, if concentration

changes cause significant alterations in the refractive index µ

of a solution, departures from Beer’s law are observed.

E.T 67

Chemical Deviations

• Apparent deviations from Beer's law are frequently

encountered as a consequence of association,

dissociation, or reaction of the absorbing species with

the solvent to generate a product that has a different

absorption spectrum with the analyte.

• A common example of this behavior is found with

acid/base indicators.

• Deviations arising from chemical factors can only be

observed when concentrations are changed.

E.T 68

Instrumental Deviations

• Unsatisfactory performance of an instrument

may be caused by fluctuations in the power-

supply voltage, an unstable light source, or a

non-linear response of the detector-amplifier

system.

E.T 69

Polychromatic Radiation

• Strict adherence to Beer’s law is observed only with truly

monochromatic radiation. This sort of radiation is only

approached in specialized line emission sources.

• All monochromators, regardless of quality and size, have a

finite resolving power and therefore minimum instrumental

bandwidth.

• the use of radiation that is restricted to a single wavelength

is seldom practical; devices that isolate portions of the

output from a continuous source produce a more or less

symmetric band of wavelengths around the desired one

E.T 70

Polychromatic Radiation (cont)

• A good picture of the effect of polychromatic

radiation can be presented as follows. When

radiation consists of two wavelengths, λ’ and λ’’, and

assuming that Beer’s law applies at each of these

individually

• the absorbance at λ’ is given by:

log ( P’o/P’ ) = A’ = ԑ’bc

P’o /P’ = 10ԑ’bc

P’ = p’0 10-ԑ’bc

E.T 71

P’’ = p0’’ 10-ԑ’’bc Similarly at λ’’ is given by

When an absorbance measurement is made with radiation composed of both

wavelengths, the power of the beam emerging from the solution is given by (P' +

P") and that of the beam from the solvent by (P’O + P’’O).

Therefore, the measured absorbance is

E.T 72

E.T 73

• If the two molar absorpitivities differ, however, the

relationship between AM and concentration will no longer be

linear; moreover, greater departures from linearity can be

expected with increasing differences between ԑ' and ԑ`'.

E.T 74

FIGURE the effect of polychromatic radiation on beer’s law. In the spectrum at the left, the

absorptivity of the analyte is nearly constant over band A from the source. Note in the beer’s law

plot at the right that using band A gives a linear relationship. In the spectrum, band B corresponds

to a region where the absorptivity shows substantial changes. In the right plot, note the dramatic

deviation from beer’s law that results.

Chapter Three

Instruments for optical spectroscopy

75 E.T

• The basic components of analytical instruments

– for emission,

– absorption and

– fluorescence spectroscopy

• Are remarkably alike in function and general performance requirements regardless of whether the instrument are designed for Uv, Vis or IR radiation.

• Called Optical instruments

76 E.T

• Spectroscopic instruments contain five

components, including:

• a stable source of radiant energy;

• a wavelength selector that permits the isolation

of a restricted wavelength region;

• a transparent container for holding the sample;

• a radiation detector or transducer, that converts

radiant energy to a usable signal (usually

electrical); and

• a signal processor and readout

77 E.T

FIGURE 1 Components for various types of instruments for

optical spectroscopy: (a) absorption spectroscopy,

(b), fluorescence and scattering Spectroscopy and (c) emission spectroscopy 78 E.T

• As can be seen in the figure, the configuration of components

(4) and (5) is the same for each type of instruments.

• Emission spectroscopy differ from the other two types in that

components 1 & 3are combined,

– that no external radiation source is required;

• the sample itself is the emitter.

– the sample is usually fed into a plasma or a flame, which provides enough

thermal energy to cause the analyte to emit characteristic radiation..

• Absorption and fluorescence spectroscopy require an external source of

radiant energy and a cell to hold the sample.

• For the absorption measurements

• The beam from the source passes through the sample after leaving the

wavelength selector.

• In fluorescence Here, two wavelength selectors are needed to select the

excitation and emission wavelengths. The selected source radiation is

incident on the sample and the radiation emitted is measured. usually at

right angles to avoid scattering.

79 E.T

Optical Material

The cell, windows, lenses and wavelength – selecting elements

in a optical spectroscopy instrument must transmit radiation in the

wavelength region being investigated.

80 E.T Figure 2a Transmittance ranges for various construction materials.

E.T 81

Figure 2b Transmittance ranges for wavelength selectors

• Ordinary silicate glass is completely adequate for use

in the visible region and has the considerable

advantage of low cost.

• In the UV region at wavelengths shorter than about

380 nm glass begins to absorb and fused silica or

quartz must be substituted.

• Also, in the IR region, glass, quartz, and fused silica

all absorb at wavelengths longer than about 2500nm.

• Hence, optical elements for IR spectrometry are

typically made from halide salts or In some cases

polymeric materials.

E.T 82

Spectroscopic Sources • To be suitable for spectroscopic studies,

– a source must generate a beam of radiation that is sufficiently powerful

to allow easy detection and measurement.

– its output power should be stable for reasonable periods of time.

• Typically, for good stability. a well-regulated power supply

must provide electrical power for the source.

• Spectroscopic sources are of two types:

• continuum sources, which emit radiation that changes in

intensity only slowly as a function of wavelength, and

• line sources, which emit a limited number of spectral lines,

each of which spans a very limited wavelength range.

• The distinction between these sources is illustrated in Figure 3.

E.T 83

84 E.T

Fig. 3

• Continuous Sources of Ultraviolet -Visible, and Near-Infrared Radiation

• Tungsten Filament Lamps

• The most common source of visible and near-infrared radiation is the tungsten filament lamp. – The energy distribution of this source approximates that of a black body and

is thus temperature dependent.

• Figure below illustrates the behaviour of the tungsten filament lamp at 3000oK.In most absorption instruments, the operating filament temperature is about 2900oK; the bulk of the energy is thus emitted in the infrared region.

85 E.T

Figure 4 (a) A tungsten lamp of the type used in spectroscopy and its spectmm

(b). Intensity of the tungsten source is usually quite low at wavelengths shorter than about 350 nm.

Note that the intensity reaches a maximum in the near-IR region of the spectrum ( -1200 nm in this case).

• A tungsten filament lamp is useful for the wavelength region between 320 and 2500 nm.

• In the visible region the energy output of a tungsten lamp varies.

– As a consequence close voltage control is required for a stable radiation source.

• Constant voltage transformers or electronic voltage regulators are often employed for this purpose

86 E.T

Hydrogen or Deuterium Lamps

• A truly continuous spectrum in the ultraviolet region is conveniently

produced by the electrical excitation of hydrogen or deuterium

at low pressure.

• The mechanism by which a continuum is produced involves the

formation of excited molecule (D2*or H2*) by absorption of

electrical energy.

• The species then dissociates to give two hydrogen or deuterium

plus ultraviolet photon.

H2 + Ee H2* H’ + H’’ + hʋ

The energy of the overall process is

Ee = E H2* = EH’ + EH’’ + hʋ

E H2* is fixed quantized energy of H2* , EH’ and EH’’

kinetic energy of H’ and H’’

87 E.T

• Most modern lamps for generating ultraviolet radiation contain deuterium and are of a low-voltage lamps. – an arc is formed between a heated, oxide-coated filament and a

metal electrode.

• The heated filament provides 'electrons to maintain a dc current when a voltage of about 40 V is applied; a regulated power supply is required for constant intensities.

• Both hydrogen and deuterium lamps produce a continuous spectrum in the region of 160 to 375 nm.

• Quartz windows must be employed in the tubes, since glass absorbs strongly in this wavelength region.

• Xenon Arc Lamps.

• This lamp produces intense radiation by the passage of current through an atmosphere of xenon.

• The spectrum is continuous over the range between about 250 and 600 nm.

88 E.T

• Continuous Sources of Infrared radiation

• The common infrared source is an inert solid heated electrically to temperatures between – 1500 and 2000oK.

• The Nernst glower is composed of Zirconium and yttrium oxides formed into a cylinder having a diameter of 2 mm and length of perhaps 20 mm,

• It emits infrared radiation when heated to a high temperature by an electric current.

• The Globar Source

• A Globar is a silicon carbide rod, usually about 50 mm in length and 5 mm in diameter.

• Radiation in the region 1 to 40 µm is emitted when the Globar heated to 15000C by passage of electricity

89 E.T

• Line Sources

• Sources that emit a few discrete lines find use in atomic

absorption spectroscopy, Raman spectroscopy, refractometry, and

polarimetry.

• Hollow Cathode Lamps

• The most common source for atomic absorption

• which consists of a tungsten anode and a cylindrical cathode

sealed in a glass tube that is filled with neon or argon at a

pressure of 1 to 5 torr

• The cathode is constructed of the metal whose spectrum is

desired or serves As a support for a coating of that metal.

90 E.T

CELLS

UV Spectrophotometer

Quartz (crystalline silica)

Visible Spectrophotometer

Glass

IR Spectrophotometer

NaCl

91 E.T

E.T 92

Figure Typical examples of commercially available cells for the

UV/visible region.

• Wavelength Selection

• Spectroscopic instruments generally required a device that

restricts the wavelength that is to be used to narrow band that

is absorbed or emitted by the analyte.

– Such device enhance selectivity and sensitivity of an instrument

• For absorption measurement, narrow bandwidths increase the

likelihood of adherence to Beer’s law.

• For this reason we usually try to select a single wavelength

where the analyte is the only absorbing species.

– Unfortunately, we cannot isolate a single wavelength of radiation from

a continuum source.

• Instead, a wavelength selector passes a narrow band of

radiation characterized by

– a nominal wavelength, an effective bandwidth, and a maximum

throughput of radiation.

93 E.T

• The effective bandwidth is defined as the width of the radiation at

half the maximum throughput.

• The ideal wavelength selector has a high throughput of radiation

and a narrow effective bandwidth.

– A high throughput is desirable because more photons pass through the

wavelength selector, giving a stronger signal with less background noise.

• A narrow effective bandwidth provides a higher resolution, with

spectral features separated by more than twice the effective

bandwidth being resolved.

94 E.T

• Generally these two features of a wavelength

selector are in opposition

• Conditions favouring a higher throughput of

radiation usually provide less resolution.

• Decreasing the effective bandwidth improves

resolution, but at the cost of a noisier signal.

• For a qualitative analysis, resolution is

generally more important than the throughput

of radiation; thus,

– smaller effective bandwidths are desirable.

• In a quantitative analysis a higher throughput

of radiation is usually desirable

95 E.T

• Wavelength Selection Using Filters

• The simplest method for isolating a narrow band of radiation is

to use an absorption or interference filter.

• Absorption filters

• work by selectively absorbing radiation from a narrow region of

the electromagnetic spectrum .

– Limited application to visible region.

• A simple example of an absorption filter usually consist of a

colored glass plate that removes part of incident radiation by

absorption.

– A purple filter, for example, removes the complementary color green

from 500–560 nm.

– Commercially available absorption filters provide effective bandwidths

from 30–250 nm.

• Interference filters are more expensive than absorption filters,

but have narrower effective bandwidths, typically 10–20 nm,

with maximum throughputs of at least 40%.

96 E.T

97 E.T

• Wavelength Selection Using Monochromators

• One limitation of an absorption or interference filter is that

they do not allow for a continuous selection of wavelength.

• If measurements need to be made at two wavelengths, then the

filter must be changed in between measurements.

• A further limitation is that filters are available for only

selected nominal ranges of wavelengths.

• An alternative approach to wavelength selection, which

provides for a continuous variation of wavelength, is the

– monochromator

• Monochromators for ultraviolet, visible, and infrared radiation

are all similar in mechanical construction in the sense that they

employ slits, lenses, mirrors, windows, and prisms or gratings.

98 E.T

• Components of Monochromator

• All monochromators contain an entrance slit, a collimating lens

or mirror to produce a parallel beam of radiation, a prism or

grating as a dispersing element and a focusing element,

• which projects a series of rectangular images of the entrance slit

upon a plane surface (the focal plane).

Polychromatic

Ray

Infrared

Red

Orange

Yellow

Green

Blue

Violet

Ultraviolet

monochromatic

Ray

SLIT

PRISM

Polychromatic Ray Monochromatic Ray

99 E.T

100 E.T

Figure Types of monochromators: (a) grating monochromator: (b) prism monochromator.

The monochromator design in (a) is a Czerny- Turner design. while the prism

monochromator in (b) is a Bunsen design. In both cases, , λ1 >λ2

• A source of radiation containing two wavelengths, λ1 and λ2, is

shown for purposes of illustration.

• This radiation enters the monochromators via a narrow

rectangular opening or slit, is collimated, and then strikes the

surface of the dispersing element at an angle.

• In the prism monochromator, refraction at the two faces results

in angular dispersal of the radiation, as shown;

• for the grating, angular dispersion results from diffraction,

which occurs at the reflective surface.

• In both designs, the dispersed radiation is focused on the focal

plane AB where it appears as two "images of the entrance slit

(one for each wavelength).

101 E.T

• Radiation detectors and Transducers

• A detector is a device that indicates the existence of some

physical phenomenon.

• Photographic film indicating presence of EMR ,pointer of

balance indicating mass d/n, mercury level in thermometer

indicating temperature change.

– Early instruments for the measurement of emission and absorption of

radiation required visual or photographic methods for detection.

• A transducer is a special type of detector that converts signals,

such as light intensity, pH, mass and temperature into electrical

signals that can be subsequently amplified,

• manipulated and finally converted into numbers that are

related to the magnitude of the original signal.

102 E.T

• Properties of Transducers

• To be useful the detector must respond rapidly to low levels of

radiant energy over a broad wavelength range.

• produce an electrical signal that can be readily amplified, and

have a relatively low noise level (for stability).

• Finally, it is essential that the signal produced be directly

proportional to the beam power P; that is,

G = KP + K'

– where G is the electrical response of the detector in units of current.

resistance, or potential. The constant K measures the sensitivity of the

detector in terms of electrical response per unit of radiant power.

– Many detectors exhibit a small constant response known as a dark

currenl K'. even when no radiation impinges on their surfaces.

103 E.T

Types of Transducers

As indicated in Table,10.4 two general types of radiation transducers

are employed; one responds to photons, the other to heat.

104 E.T

• All photon detectors are based upon interaction of

radiation with a reactive surface to produce electrons

(photoemission) or

• to promote electrons to energy states in which they

can conduct electricity (photoconduction).

• Only ultraviolet, visible, and near-infrared radiations

have sufficient energy to cause these processes to

occur.

• Photoelectric detectors also differ from heat detectors

in that their electrical signal results from a series of

individual events (absorption of one photon).

105 E.T

• Photon Detectors

• Several types of photon detectors are available. including:

– photovoltaic cells, phototubes, photomultiplier tubes, semiconductor

detectors, silicon diodes and charge –transfer device

• Phototubes

• which consists of a semi cylindrical cathode and a wire

anode sealed inside an evacuated transparent envelope.

• The concave surface of the electrode supports a layer of

photoemissive material that tends to emit electrons upon

being irradiated.

• When a potential is applied across the electrodes, the

emitted electrons flow to the wire anode, generating a

photocurrent.

• The currents produced are generally about one-tenth as great as

those from a photovoltaic cell for a given radiant intensity.

106 E.T

E.T 107

Figure A phototube and accompanying circuit. The photocurrent induced by

the radiation causes a voltage across the measuring resistor:

this voltage is then amplified and measured.

• The number of electrons ejected from a

photoemissive surface is directly proportional

to the radiant power of the beam striking that

surface.

• As the potential applied across the two

electrodes of the tube is about 90 V, the

fraction of the emitted electrons reaching the

anode to give current that is proportional to the

radiant power.

• Assignment Discuss in detail for the remaining

four type of detectors

108 E.T

• Heat detector

• Infrared radiation generally does not have sufficient energy to

produce a measurable current when using a photon transducer.

– A heat detector, therefore, is used for infrared spectroscopy.

• The absorption of infrared photons by a heat detector increases

its temperature, changing one or more of its characteristic

properties.

• The pneumatic transducer, for example consists of a small tube

filled with xenon gas equipped with an IR-transparent window

at one end, and a flexible membrane at the other end.

• A blackened surface in the tube absorbs photons, increasing

the temperature and, therefore, the pressure of the gas.

• The greater pressure in the tube causes the flexible membrane

to move in and out, and this displacement is monitored to

produce an electrical signal.

109 E.T

• SIGNAL PROCESSORS AND READOUTS

• The signal processor is ordinarily an electronic

device that amplifies the electrical signal from

the detector; in addition, it may alter the signal

from dc to ac (or the reverse)

– change the phase of the signal, and filter it to

remove unwanted components.

• Furthermore, the signal processor may be

called upon to perform such mathematical

operations on the signal as differentiation

integration, or conversion to a logarithm.

110 E.T

Chapter Four

Atomic Absorption and Emission Spectroscopy

111 E.T

ATOMIC SPECTROSCOPY

• Elemental analysis at the trace or ultratrace level can be

performed by a number of analytical techniques; however, atomic spectroscopy remains the most popular approach.

• Atomic spectroscopy can be subdivided into three fields:

• Atomic emission spectroscopy (AES),

• Atomic absorption spectroscopy (AAS), and

• Atomic fluorescence spectroscopy (AFS)

• that differ by the mode of excitation and the method of measurement of the atom concentrations.

112 E.T

• The selection of the atomic spectroscopic technique to be used for a particular application should be based on the desired result, since each technique involves different measurement approaches.

• AES excites ground-state atoms and then quantifies the concentrations of excited-state atoms by monitoring their special deactivation.

• AAS measures the concentrations of ground-state atoms by quantifying the absorption of spectral radiation that corresponds to allowed transitions from the ground to excited states.

• AFS determines the concentrations of ground-state atoms by quantifying the radiative deactivation of atoms that have been excited by the absorption of discrete spectral radiation.

113 E.T

114 E.T

Flame Atomic Emission

Spectroscopy • Introduction Flame photometry, now more properly called

flame atomic emission spectrometry, is a relatively old instrumental analysis method.

• Atomic emission is a fast, simple, and sensitive method for the determination of trace metal ions in solution.

• Because of the very narrow (ca. 0.01 nm) and characteristic emission lines from the gas-phase atoms in the flame plasma, the method is relatively free of interferences from other elements.

• The method is suitable for many metallic elements, especially for those metals that are easily excited to higher energy levels at the relatively cool temperatures of typical flames – Na, K, Ca, Rb, Cs, Cu, and Ba.

115 E.T

Theory

• Sample solution sprayed or aspirated as fine mist into flame. • Conversion of sample solution into an aerosol by atomizer

(scent spray) principle. – in this stage no chemical change in the sample

• Heat of the flame vaporizes sample constituents – Solvent vaporizes leaves solid partial of salt

• By heat of the flame + action of the reducing gas (fuel), molecules & ions of the sample species are decomposed and reduced to give ATOMS. – eg Na+ + e- --> Na

• Heat of the flame causes excitation of some atoms into higher electronic states.

• Excited atoms revert to ground state by emission of light energy, of characteristic wavelength; measured by detector.

116 E.T

E.T 117

• Flame Emission -> it measures the radiation

emitted by the excited atoms that is related to

concentration.

• Atomic Absorption -> it measures the radiation

absorbed by the unexcited atoms that are

determined.

• Atomic absorption depends only upon the number

of unexcited atoms, the absorption intensity is not

directly affected by the temperature of the flame.

• The flame emission intensity in contrast, being

dependent upon the number of excited atoms, is

greatly influenced by temperature variations.

118 E.T

Instrumentation:

• Flame photometer consists of following components:

• Pressure regulators and flow meters for fuel gases

• Flame source

• The atomizing device

• Wavelength selectors: May be a filter or Monochromator

• Photosensitive detectors

• Read out unit.

119 E.T

120 E.T

121 E.T

• Requirements of flame:

• It should have proper temperature

• Temp. should remain constant throughout operation.

• There should not be any fluctuation during burning.

• Function flame includes:

– To convert the constituent of the liquid sample in to vapor state.

– To decompose the constituent into atoms or simple molecules.

– To electronically excite a fraction of the resulting atomic or molecular spectra.

122 E.T

• Transformation of sample into vapour

• With a pneumatic nebulizer operated by a compressed

gas, the solution is aspirated from the sample container

and nebulized into a mist or aerosol of fine droplets.

• By desolvation, i.e., evaporation of the solvent from the

droplets, this mist is converted into a dry aerosol which

is volatilized in the flame.

• The atomization, i.e., the conversion of volatilized

analyte into free atoms is performed by the flame or

other atomizer.

123 E.T

• Nebulizers used in flame photometer

• According to the source of energy used for

nebulization as, for example, Pneumatic or

ultrasonic nebulizers.

• According to the relative position of the

capillaries for the nebulizing gas and the

aspirated liquid, e.g., angular and concentric

nebulizers

124 E.T

125 E.T

• Burners

• Flames are produced by means of a burner to which fuel and oxidant are supplied in the form of gases.

• With the premix burner, fuel and oxidant are thoroughly mixed inside the burner housing before they leave the burner ports and enter the primary combustion or inner zone of the flame.

• This type of burner usually produces an approximately laminar flame, and is commonly combined with a separate unit for nebulizing the sample.

• The direct-injection burner combines the function of nebulizer and burner.

126 E.T

127 E.T

128 E.T

• Thermal energy in flame atomization is provided by the combustion of a fuel–oxidant mixture.

• Common fuels and oxidants and their normal temperature ranges are listed in Table 28.2. Of these, the air–acetylene and nitrous oxide-acetylene flames are used most frequently.

129 E.T

• Flame temperature:

• The optimum flame temp. depends upon several factors as excitation energy of the element, sensitivity of the measurement, Presence of other elements etc.

• The temp. of flame lies between 1000OC and 3000OC.

• Mixture of coal gas and air do not give very hot flames, because of presence of nitrogen.

• The cyanogens gas produces excellent spectra but it is toxic.

• Acetylene and hydrogen are most frequent choice

130 E.T

Atomic Absorption Spectroscopy （AAS）

• Atomic absorption spectroscopy is a quantitative method of

analysis that is applicable to many metals and a few

nonmetals.

• The technique was introduced in 1955 by Walsh in Australia

(A.Walsh, Spectrochim. Acta, 1955, 7, 108)

• A much larger number of the gaseous metal atoms will

normally remain in the ground state.

• These ground state atoms are capable of absorbing radiant

energy of their own specific resonance wavelength.

• If light of the resonance wavelength is passed through a flame

containing the atoms in question, then part of the light will be

absorbed.

• The extent of absorption will be proportional to the number of

ground state atoms present in the flame.

131 E.T

132 E.T

133 E.T

Instrumentation

134 E.T

135 E.T

136 E.T

137 E.T

138 E.T

139 E.T

140 E.T

141 E.T

142 E.T

143 E.T

144 E.T

145 E.T

146 E.T

147 E.T

148 E.T

149 E.T

150 E.T

151 E.T

Application of Flame photometer

• Determination of Na+, K+, Ca++,Mg++ in

biological fluids (Serum, plasma & Urine),

Analysis of Industrial waste for pollutants

Hardness of water

• Determination of Heavy metal at trace and

ulteratrace level

E.T 152

Chapter Five

UV-Visible spectroscopy

153 E.T

154 E.T

UV-Visible spectrum

• The part of the electromagnetic radiation

spectrum that you are most familiar with is

"visible light" but

• this is just a small portion of all the possible

types – Violet :400 - 420 nm

– Indigo : 420 - 440 nm

– Blue : 440 - 490 nm

– Green : 490 - 570 nm

– Yellow : 570 - 585 nm

– Orange :585 - 620 nm

– Red : 620 - 780 nm 155 E.T

Ultraviolet and visible spectroscopy

• It is used to measure the multiple bonds or atomic

conjugation within the molecule.

• The UV-Visible region is subdivided as below

– Vacuum UV: 100-200 nm

– Near UV: 200 to 400 nm

– Visible region: 400 to 750 nm

• Vacuum UV is so named because molecule of air

absorb radiation in these region.

• The radiation is assessable only in special

vacuum equipments. 156 E.T

Basics of UV Light Absorption

• Ultraviolet/visible spectroscopy involves the absorption of ultraviolet/visible light by a molecule

• causing the promotion of an electron from a ground electronic state to an excited electronic state

• Absorption of this relatively high-energy light causes electronic excitation.

• The easily accessible part of this region (wavelengths of 200 to 800 nm)

• absorption only if conjugated pi-electron systems are present.

157 E.T

• The visible region of the spectrum comprises photon energies of 36 to 72 kcal/mole, and

• the near ultraviolet region, out to 200 nm, extends this energy range to 143 kcal/mole.

• Ultraviolet radiation having wavelengths less than 200 nm is difficult to handle, and is seldom used as a routine tool for structural analysis

• The energies are sufficient to promote or excite a molecular electron to a higher energy orbital.

• The absorption spectroscopy carried out in this region is sometimes called "electronic spectroscopy".

158 E.T

• Electronic transitions

– d and f electrons

– Charge transfer reactions

• p, s, and n (non-bonding) electrons

159 E.T

Absorption Involving d and f Electrons

• Most transition-metal ions absorb in the

ultraviolet or visible region of the spectrum.

• For the lanthanide and actinide series, the

absorption process results from electronic

transitions of 4f and 5f electrons;

• for elements of the first and second transition-

metal series, the 3d and 4d electrons are

responsible for absorption.

160 E.T

161 E.T

D transitions

• Partially occupied d orbitals

– Transitions from lower to higher energy levels

• Splitting of levels due to spatial distribution

similar

Axial direction

162 E.T

D transitions

• Binding ligands on axis have greater effect

on axial orbitals

163 E.T

D transitions

• ∆ value dependent upon ligand field

strength

–

Actinide transitions

400 500 600 700 8000

1

2

3

4

5

Ab

so

rba

nce

Wavelength (nm)

Normal

Heavy

Light

Pu4+

(489 nm)

Pu6+

(835 nm)

Figure 2: UV-vis spectra of organic phases for

13M HNO3 system 165 E.T

Charge-transfer Transitions

• Electron donor and acceptor characteristics

– Absorption involves e- transitions from donor to

acceptor

• SCN to Fe(III)

– Product is Fe(II) and neutral SCN

– Metal is acceptor

• An exception is the 1,10--phenanthroline complex of iron(II) or copper(I),

where the ligand is the acceptor and the metal ion is the donor.

166 E.T

• p, s, and n (non-bonding) electrons

167 E.T

Sigma and Pi orbitals

168 E.T

Electronic transactions involved

169 E.T

170 E.T

Electronic spectroscopy

• Out of the six transitions outlined, only the two

lowest energy ones (left-most, colored blue)

are achieved by the energies available in the

200 to 800 nm spectrum.

• As a rule, energetically favored electron

promotion will be from the highest occupied

molecular orbital (HOMO) to the lowest

unoccupied molecular orbital (LUMO), and

the resulting species is called an excited state

171 E.T

Types of Transitions

• There are several types of electronic transitions

available to a molecule including:

• σ to σ * (alkanes)

• σ to π * (carbonyl compounds)

• π to π * (alkenes, carbonyl compounds,

alkynes, azo compounds)

• n to σ * (oxygen, nitrogen, sulfur, and halogen

compounds)

• n to π * (carbonyl compounds)

172 E.T

σ σ * Transitions

• An electron in a bonding σ orbital is excited to the corresponding antibonding orbital. • The energy required is large.

• For example, alkanes as methane (which has only C-H bonds, and can only undergo σ σ * transitions)

• shows an absorbance maximum at 125 nm.

• Absorption maxima due to σ σ * transitions are not seen in typical UV-Visible spectra (200 - 700 nm).

• σ bonds are very strong and requires higher energy of vacuum UV.

173 E.T

s smax = 135 nm (a high energy transition)

Absorptions having max < 200 nm are difficult to observe because everything (including quartz glass and air) absorbs in this spectral region.

Ethane

174 E.T

nσ * Transitions

• Saturated compounds containing atoms with lone pairs (non-bonding electrons) are capable of n σ * as transitions.

• These transitions usually need less energy than σ σ * transitions.

• They can be initiated by light whose wavelength is in the range 150 - 250 nm.

• The number of organic functional groups with n σ * peaks in the UV region is small.

• These transitions are involved in saturated compound with one hetero atom with unshared pair of electron i.e. saturated halides, ethers, aldehydes, ketones, amines etc.

• These transitions are sensitive to hydrogen bonding eg alcohol and ethers which absorbs at wavelength shorter than 185 nm therefore used as a solvent in UV

175 E.T

π π * Transitions

• For molecules that possess π bonding as in alkenes, alkynes, aromatics, acyl compounds or nitriles,

• energy that is available can promote electrons from a π Bonding molecular orbital to a π Antibonding molecular orbital (π * ).

• This is called a π π * transition.

• The absorption peaks for these transitions fall in an experimentally convenient region of the spectrum (200 - 700 nm).

• These transitions need an unsaturated group in the molecule to provide the π electrons.

176 E.T

C C

s

s

hv

p

p

s

s

p

p

p pExample: ethylene absorbs at longer wavelengths:

= hv =hc/

177 E.T

n π * Transitions

• Lone pairs that exist on Oxygen atoms and Nitrogen atoms may be promoted from their non-bonding molecular orbital to a π antibonding molecular orbital within the molecule.

• This is called an n π * transition and

• requires less energy (longer wavelength) compared to a π π * transition within the same chromophore.

• These are available in compounds with unsaturated centers. eg. Alkenes. They requires lowest energy as compare to others

178 E.T

s

s

hv

p

p

n

s

s

p

p

n

C O

pn

179 E.T

C C

HOMO

LUMO

Conjugated systems:

Preferred transition is between Highest Occupied Molecular Orbital

(HOMO) and Lowest Unoccupied Molecular Orbital (LUMO).

Note: Additional conjugation (double bonds) lowers the HOMO-LUMO energy gap: Example: 1,3 butadiene: max = 217 nm ; = 21,000 1,3,5-hexatriene max = 258 nm ; = 35,000

180 E.T

O

O

O

Similar structures have similar UV spectra:

max = 238, 305 nm max = 240, 311 nm max = 173, 192 nm

181 E.T

E.T 182

Table: absorption characteristics of some common chromophores

E.T 183

Activity one:

Transitions n-> σ *, π -> π*, n->π* , σ -> σ*, σ ->π*

Choose possible transitions from the above list for each

chromophores.

R-OH n-> σ *

R-O-R n-> σ *

R-CH2-R σ -> σ*

R-C≡C-R π -> π*

R-C≡N n->π*, π -> π*

R-CHO π -> π*, n->π*

RCOOH n->π*

Instrumentation: :

184 E.T

Figure: Instrumental designs for UV-visible photometers or spectrophotometers. In (a), a single-beam instrument is shown. Radiation from the filter or monochromator passes through either the reference cell or the sample cell before striking the photodetector. In (b), a double-beam-in-space instrument is shown. Here, radiation from the filter or monochromator is split into two beams that simultaneously pass through the reference and sample cells before striking two matched photodetectors. In the double-beam-in-time instrument (c), the beam is alternately sent through reference and sample cells before striking a single photodetector. only a matter of milliseconds separate the beams as they pass through the two cells.

185 E.T

Instrumentation: • A beam of light from a visible and/or UV light source

is separated into its component wavelengths by a

prism or diffraction grating.

• Each monochromatic (single wavelength) beam in

turn is split into two equal intensity beams by a half-

mirrored device.

• One beam, the sample beam , passes through a small

transparent container (cuvette) containing a solution

of the compound being studied in a transparent

solvent.

• The other beam, the reference , passes through an

identical cuvette containing only the solvent. 186 E.T

• The intensities of these light beams are then measured

by electronic detectors and compared.

• The intensity of the reference beam, which should

have suffered little or no light absorption, is defined

as I0.

• The intensity of the sample beam is defined as I.

• Over a short period of time, the spectrometer

automatically scans all the component wavelengths in

the manner described.

• The ultraviolet (UV) region scanned is normally from

200 to 400 nm, and the visible portion is from 400 to

800 nm.

187 E.T

Solvent effect:

• Different compounds may have very different

absorption maxima and absorbances.

• Intensely absorbing compounds must be examined in

dilute solution, so that significant light energy is

received by the detector, and this requires the use of

completely transparent (non-absorbing) solvents.

• The most commonly used solvents are water,

ethanol, hexane and cyclohexane.

• Solvents having double or triple bonds, or heavy

atoms (e.g. S, Br & I) are generally avoided.

188 E.T

• Solvents must be transparent in the region to

be observed; the wavelength where a solvent

is no longer transparent is referred to as the

cutoff

189 E.T

Table: solvents for the UV and Visible regions

190 E.T

• H-bonding further complicates the effect of vibrational and

rotational energy levels on electronic transitions, dipole-

dipole interacts less so

• The more non-polar the solvent, the better (this is not

always possible)

Figure: effect of solvent on the absorption spectrum of acetaldehy

191

The Spectrum

1. The x-axis of the spectrum is in wavelength; 200-350 nm for UV,

200-700 for UV-VIS determinations

2. Due to the lack of any fine structure, spectra are rarely shown in their

raw form, rather, the peak maxima are simply reported as a numerical

list of “lambda max” values or λmax

λmax = 206 nm

252

317

376

O

NH2

O

E.T

192

The Spectrum

1. The y-axis of the spectrum is in absorbance, A

2. From the spectrometers point of view, absorbance is the inverse of

transmittance: A = log10 (p0/p)

3. From an experimental point of view, three other considerations must

be made:

i. a longer path length, l through the sample will cause

more UV light to be absorbed – linear effect

ii. the greater the concentration, c of the sample, the more

UV light will be absorbed – linear effect

iii. some electronic transitions are more effective at the

absorption of photon than others – molar absorptivity, ε

this may vary by orders of magnitude…

192 E.T

193

4. These effects are combined into the Beer-Lambert Law: A = ε c l

i. for most UV spectrometers, l would remain constant (standard

cells are typically 1 cm in path length)

ii. concentration is typically varied depending on the strength of

absorption observed or expected – typically dilute – sub .001

M

iii. molar absorptivities vary by orders of magnitude:

• values of 104-106 are termed high intensity absorptions

• values of 103-104 are termed low intensity absorptions

• values of 0 to 103 are the absorptions of forbidden

transitions

A is unit less, so the units for ε are cm-1 · M-1

5. Since path length and concentration effects can be easily factored

out, absorbance simply becomes proportional to ε, and the y-axis is

expressed as e directly or as the logarithm of ε

E.T

E.T 194

The Spectrum

195

Terminology Chromophore: • Chromophore: A covalently unsaturated group

responsible for electronic absorption.

• Or Any group of atoms that absorbs light whether or not

a color is thereby produced. Eg C=C, C=O, NO2 etc

• A compound containing chromophore is called

chromogen.

• There are two types of chromophore

– Independent chromophore: single chromophore is sufficient to

import color to the compound eg. Azo group

– Dependent chromophore: When more then one chromophore is

required to produce color.

• Eg acetone having 1 kentone group is colorless where as diacetyl

having two kentone group is yellow

E.T

Auxochrome:

• A saturated group with non bonding electron when

attached to chromophore alters both wavelengths as

well as intensity of absorption. Eg OH, NH2, NHR

etc.

OR

• A group which extends the conjugation of a

chromophore by sharing of nonbonding electrons.

196 E.T

Substituent Effects

• General – Substituents may have any of four

effects on a chromophore

• Bathochromic shift (red shift) – a shift to

longer λ; lower energy

• Hypsochromic shift (blue shift) – shift to

shorter λ; higher energy

• Hyperchromic effect – an increase in intensity

• Hypochromic effect - a decrease in intensity

197 E.T

200 nm 700 nm

H

ypochro

mic

Hypsochromic

Hyperc

hro

mic

Bathochromic

E.T

199

• Substituent Effects

1. Conjugation – most efficient means of bringing about a

bathochromic and hyperchromic shift of an unsaturated

chromophore:

H2CCH2

-carotene

O

O

max nm 175 15,000

217 21,000

258 35,000

n p* 280 27 p p* 213 7,100

465 125,000

n p* 280 12 p p* 189 900

E.T

200

• Substituent Effects

2. Conjugation – Alkenes

Extending this effect out to longer conjugated systems the

energy gap becomes progressively smaller:

Energy

ethylene

butadiene

hexatriene

octatetraene

Lower energy = Longer wavelengths

E.T

Activity 1

What type of shift observed in 1&2,3&5 and

4&6

• max ᵋmax 1 .CH3-COH 293 nm 12

2. CH3-COCl 235 nm 53

E.T 201

5. n p* 280 27 6. p p* 213 7,100

3. n p* 280 12 4. p p* 189 900

Hypsoc

hromic

&

Hyperc

hromic

Bathoc

hromic

&

Hyperc

hromic

Hyperc

hromic 3&5 4&6

202

• We will find that the effect of substituent groups can be reliably

quantified from empirical observation of known conjugated

structures and applied to new systems

• This quantification is referred to as the Woodward-Fieser Rules

which we will apply to two specific chromophores:

1. Conjugated dienes

2. Aromatic systems

E.T

A. Dienes

1. General Features

For acyclic butadiene, two conformers are possible – s-cis and s-trans

s-trans s-cis

The s-cis conformer is at an overall higher potential energy than the s-trans;

therefore the HOMO electrons of the conjugated system have less of a jump to

the LUMO – lower energy, longer wavelength

203

A. Dienes 1. General Features Two possible p p* transitions can occur for butadiene Y2 Y3

and Y2 Y4

*

The Y2 Y4

* transition is not typically observed:

• The energy of this transition places it outside the region typically observed – 175 nm

• For the more favorable s-trans conformation, this transition is forbidden

The Y2 Y3* transition is observed as an intense absorption

s-trans s-cis

175 nm –forb.

217 nm 253 nm

175 nm

Y4

Y2

Y1

Y3

E.T

204

A. Dienes

1. General Features The Y2 Y3

* transition is observed as an intense absorption ( =

20,000+) based at 217 nm within the observed region of the UV While this band is insensitive to solvent (as would be expected) it is

subject to the bathochromic and hyperchromic effects of alkyl substituents as well as further conjugation

Consider:

max = 217 253 220 227 227 256 263 nm

E.T

205

A. Dienes 2. Woodward-Fieser Rules - Dienes The rules begin with a base value for max of the chromophore being

observed: acyclic butadiene = 217 nm The incremental contribution of substituents is added to this base value

from the group tables:

Group Increment

Extended conjugation +30

Each exo-cyclic C=C +5

Alkyl +5

-OCOCH3 +0

-OR +6

-SR +30

-Cl, -Br +5

-NR2 +60 E.T

206

A. Dienes

2. Woodward-Fieser Rules - Dienes For example: Isoprene - acyclic butadiene = 217 nm one alkyl subs. + 5 nm 222 nm Experimental value 220 nm Allylidenecyclohexane - acyclic butadiene = 217 nm one exocyclic C=C + 5 nm 2 alkyl subs. +10 nm 232 nm Experimental value 237 nm

E.T

E.T 207

Calculate wavelengths for the fo